Dark Side
by HeMustBeMagic
Summary: What if Stiles was infested with a demon that comes out every night after a full moon? What if this demon was responsible for murders and even more heinous crimes? What if Derek was the only one who knew what was going on and Stiles had no idea? And what if, the sour wolf was the only one Stiles could trust with his life, and his future? Everyone has a darkside. M Rating. Sterek.


Chapter One: When Life Gives You Lemons

It was times like this that Derek felt sick. The day always seemed to drag on after the night after a full moon. He should have, in all aspects, been more focused on the night OF a full moon. Honestly; that would have been a more reasonable worry. After all, that would fit the persona. Care about himself, care nothing about others. That was the view that people had for him for the longest time. What did Stiles call him? A 'Sour Wolf'? The term peeved him just thinking about it, causing his lip to twitch slightly in a feeling of disgust. The brewing feeling of anger that boiled inside of him quickly settled. Anger wasn't what he needed right now. If anything he needed to stay calm. As calm as fucking possible.

He hated taken Stiles truck that day to prevent anyone from asking any questions; or any more than usual. He just hoped Scott wasn't putting anything together. It was the third time that Stiles had gone 'missing' after a full moon and he could tell on the faces of the pack, things were being put together in those empty headed brains of theirs. He wasn't sure how much longer he could keep this going, how much longer he could protect Stiles before the others needed to be involved. A small part of Derek, a very small part, actually wanted them to know; so he didn't have to burden this alone. He had enough to deal with and this shit wasn't on his list of things that 'Derek gives a shit about'. But when life gives you lemons.

"Throw them at people." He muttered, slamming the door to the truck after driving down a dirt road, turning left, driving another mile, and then down a hill. The location where he parked was off the path of any map and his cellphone was even having trouble getting reception. Something about the static voice on the end of the line when Scott called him panicked last month gave him comfort. No one could find him or Stiles here. That was best for both of them. The alpha gave a look left, then a look right before straightening his jacket and giving a dash, jogging slowly. He still had a few miles to go.

The jog, relaxing pace and leisurely gave him time to think. The juxtaposition of the situation; the fact the scenery was so beautiful, yet where he was running to was pure darkness caused him to brood even more. How the hell could this happen? That was a question he asked himself all the time; more often than he wanted to admit. It made no logical sense and a lot of things made sense to Derek. But this? Insane wasn't even a word close to cutting it. Of all people for it to happen to Stiles was a cruel turn of fate. Derek didn't care really. Well, he didn't care as much as other people in the pack cared. He cared because Stiles was useful. He could figure out where Scott was usually faster than Derek could. He was doing this because to help Scott, he would need to help Stiles.

Derek though, wasn't comfortable with the amount of times he told himself that over and over again.

In a matter of minutes Derek reached the destination through the weaving winding woods. The small location was a cave, or at least it should have been a cave. In all honesty it looked like an overgrow thicket. The rocks had water dripping from them thanks to the recent rain and a small smell of what could only be described as mold seemed to be waffling from the depths of the cavern. The sun, which was now high in the sky in the blue and beautiful day, couldn't penetrate the depths of the cave. About 24 feet or so down and Derek's eyes itched to assist him in a supernatural way. He already knew what was down there though, he didn't need any help.

For a moment or two, in order to gather his thoughts Derek stood. This was all new to him. He had no idea what to expect or how to deal with what was down there. With a cold expression, the alpha flexed his hands into fist before letting his boots crunch the ground, sinking slightly into the moss and started to walk. He let the cave affect his senses, the walls bouncing off sounds like a ping pong table. He didn't care that the darkest and the chill slowly started to creep even over his augmented skin. A part of him knew it wasn't really the cold and actually his own mind playing tricks on him. That was another thing Derek didn't like. Since when did his mind play tricks on him?

The deeper he went into the cave the darker it became and he knew he needed to use his vision. A part of him didn't want to see what he found in there. Technically though, it wasn't what he found. He had put it there for safe keeping. It? Yeah, it was the right word. 100 steps in, Derek stopped, took a small breath and blinked, letting his eyes take over and reveal to him what he didn't want to see.

Chained to the wall, suspended with his feet barely above the ground Stiles hung. His wrist were wrapped three times with chained which were looped and bolted into the stone. His waist, neck and ankles were bound the same away and Derek could tell from the sight and smell of the iron liquid that stained the boys thin frame, he struggled to get out. His eyes scanned over the man, sniffing the air. Besides the cuts on his arms and body from straining to much, the boy seemed fine. Oh, and the bruises. Those were Derek's doing. He'd apologize later.

For a moment though, Derek's heart skipped a beat, wondering if the other was alive. His body was motionless and his breath was so shallow he didn't seem to be moving. Approaching the other didn't seem the wisest of moves, but he had to know. If Stiles was dead…well that would be a real shit storm.

Carefully, Derek took one small step, then another. No reaction. He followed up with a few more steps and still no reaction. When he reached a few feet from the boy's face, ready to make another step though, Stiles jerked away. His body snapped forward, his slender yet fit muscles bulged under his chains as he lunged forward. Thankfully, Derek had made sure his knots were tight but not tight enough it seemed. He barely moved back in time before Stiles chomped, his teeth narrowly missing the man's ear.

"Pity." Stiles purred, shrugging. "Thought I had you that time." He sighed. "You really should have left me something to eat you know. A rabbit or something. Would have made the time go by faster. Props on the chains though, well done."

He wasn't sure what was more disgusting; the fact he had to beat Stiles, drag him into the cave and tie him up like this…or the fact Stiles wasn't himself. No he was far from himself. Stiles didn't have pitch black eyes, not usually at least. Stiles wouldn't have blood pooling from his mouth and staining his lips and the Stiles he knew wouldn't have to be chained to a wall. This is why he called it an 'it' and not a 'he'. He said nothing but watched. The 12 hours were almost over, that was all that mattered. Once that was done he'd deal with the issue of explaining why he looked like a rag doll thrown off a cliff. One issue at a time though.

"What, not going to say anything sour wolf?" Stiles purred. "No greeting? It's been ever so long since I've seen you."

"Bite me."

"You first."

Derek scowled, glaring at Stiles in the dark. "You know I can see you." Stiles said back, cocking his head to the side. "You're not the only one who can see in the night, sour wolf. Though this body is nothing compared to yours. Should have found a way to get into you instead. This one screams far too much. A bit nosey for my taste and I've been in some nosey people." Stiles spoke slowly and shrugged.

Derek ignored the goading words as much as possible. "Then why don't you just fucking leave? Seems simple enough."

"But not as much fun." He retorted. "And besides, it's what I got. What's that expression? When life gives you lemons make lemonade."

"I'd prefer you just choke on them." Derek shot back.

A patronizing painful pout appeared on Stiles face before a smirk appeared, showing off a row of teeth that seemed, but Derek couldn't confirm sharper than before. "Not your best retort, pup but good enough. How about you undo these chains and I'll give you something to choke on?"

It took all Derek had to NOT shout something back at the boy. Stiles weren't being himself because Stiles WASN'T himself. A frustrated flare of his nostrils caused him to growl out and only mutter a question. "It's about time, isn't it? 12 hours this time, right?" He asked.

That seemed to yield the reaction he was looking for. That all high and mighty smirk disappeared for a moment on the boy's lips before Stiles scowled. "The dog can count, good job. But yes, 12 hours this time; 24 next month. Hopefully we'll be able to talk a little more. I do enjoy the time we spend together."

"You mean when you aren't responsible for 3 brutal murders in one night?"

"Semantics." Stiles shrugged. His eyes drifted down to Derek's phone in his pocket. For some reason though, Derek had no doubt thinking Stiles already knew what time it was. "Same time next month?"

Derek grimaced. "Wouldn't miss it for the world."

A final smirk took over Stiles body before he fell limp. Derek caught his breath. For brief second Stiles was dead. No warmth, no heartbeat no breathing until it seemed the boy broke through the surface. He gasped out for breath, body twitching for a moment in surprise. Without missing a beat Derek moved forward, starting to undo the chains.

"You're fine. "He simply said.

"What happened?" Stiles asked with a pained and confused voice.

That, wasn't something Derek could answer. Silence permeated the cave for a moment which prompted Stiles to ask again.

"I said you're fine." Derek repeated. It was all he could say. In truth he didn't know.

He wasn't sure he wanted to know.

A/N: **I DO NOT OWN THE TEEN WOLF IDEA, CHARACTERS, OR LOCATION. **

Another Sterek Fic! Oh My! Well this one is darker and I thank everyone who read it. A little confusing? maybe? Maybe not? Well if it is next chapter will explain a lot hopefully. i do warn you for those not interested there will be some OC characters in here of my creation who will have a pretty heavy existence. They are more used for plot development and pushing this OC concept forward. Hope you enjoyed it. This story will be fun to write and action packed! See you next week!


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